Interpretive Summary: Arid and semi-arid regions account for approximately one-third of the land mass of earth. These regions are experiencing continued pressure from population growth in many parts of the world. Water is a critical resource in these regions and is often in short supply. Detailed study of water resources and the hydrology of semi-arid regions is important if we are to continue to populate and use these regions. Rainfall estimates from National Weather Service radar shown daily on popular news forecasts are being used for water resource decisions and models. However, the methods to estimate rainfall from radar are not well tested for semi-arid regions. Rainfall observations from the Walnut Gulch Experimental Watershed, operated by the U.S. Dept. of Agriculture, Agricultural Research Service were used to better describe and understand the nature of the methods used to transform radar into rainfall estimates. It was found that the standard method used by the National Weather Service greatly overestimates rainfall as compared to rain measured in ground-based rain gauges. This in turn can result in significant overestimates of flood waters. The results of this study suggest that the standard National Weather Service method, used nationally for convective storms, might be substantially improved for regional flood forecasting.

Technical Abstract:
Radar-based estimates of rainfall rates and accumulations are one of the principal tools used by the National Weather Service (NWS) to identify areas of extreme precipitation that could lead to flooding. Radar-based rainfall estimates have been compared to gauge observations over a densely instrumented, experimental watershed to derive an accurate reflectivity-rainfall rate (i.e., Z-R) relationship for these events. The resultant Z-R relationship, which is much different than the NWS operational Z-R, has been examined for a separate, independent event that occurred over a different location. For all events studied, the NWS operational Z-R significantly overestimates rainfall compared to gauge measurements. The gauge data from the experimental network, the NWS operational rain estimates, and the improved estimates resulting from this study have been input to a hydrologic model to "predict" watershed runoff for an intense event. Rainfall data from the gauges and from the derived Z-R relation produce predictions very close to observed stream flow. The NWS Z-R estimates lead to a predicted discharge 300 percent larger than that observed. The experimentally-derived Z-R relationship may provide more accurate radar estimates for convective storms over the southwest U.S. than does the operational Z-R used by the NWS. These initial results suggest that the generic NWS Z-R relation, used nationally for convective storms, might be substantially improved for regional flood forecasting.